1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a method and system for utilizing secondary-side conduction time parameters of a switching power converter to provide energy to a load.
2. Description of the Related Art
Many electronic systems utilize switching power converters to efficiently convert power from one source into power useable by a device (referred to herein as a “load”). For example, power companies often provide alternating current (AC) power at specific voltages within a specific frequency range. However, many loads utilize power at a different voltage and/or frequency than the supplied power. For example, some loads, such as light emitting diode (LED) based lamps operate from a direct current (DC). “DC current” is also referred to as “constant current”. “Constant” current does not mean that the current cannot change over time. The DC value of the constant current can change to another DC value. Additionally, a constant current may have noise or other minor fluctuations that cause the DC value of the current to fluctuate. “Constant current devices” have a steady state output that depends upon the DC value of the current supplied to the devices.
LEDs are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output, long life, and environmental incentives such as the reduction of mercury. LEDs are semiconductor devices and are best driven by direct current. The brightness of the LED varies in direct proportion to the DC current supplied to the LED. Thus, increasing current supplied to an LED increases the brightness of the LED and decreasing current supplied to the LED dims the LED.
FIG. 1 depicts power distribution system 100 that converts power from voltage source 102 into power usable by load 104. Load 104 is any type of load, such as a load that includes one or more LEDs. A controller 106 controls the power conversion process. Voltage source 102 is any voltage source such as a rectified alternating current (AC) input voltage or a DC voltage source. In at least one embodiment, the voltage source 102 is, for example, a public utility, and the AC voltage VIN is, for example, a 60 Hz/110 V line voltage in the United States of America or a 50 Hz/220 V line voltage in Europe. The switching power converter 110 serves as a power supply that converts the AC voltage VX into a DC link voltage VLINK.
The controller 106 provides a control signal CS1 to control conductivity of the current control switch 112 of flyback-type switching power converter 110 to control the conversion of the input voltage VIN into a secondary voltage VS. When control signal CS1 causes switch 112 to conduct, a primary-side current iPRIMARY flows into a primary coil 114 of transformer 116 to magnetize the primary coil 114. When control signal CS1 opens switch 112, primary coil 114 demagnetizes. The magnetization and demagnetization of the primary coil 114 induces a secondary voltage VS across a secondary coil 118 of transformer 116. Primary voltage VP is N times the secondary voltage VS, i.e. VP=N·VS, and “N” is a ratio of coil turns in the primary coil 114 to the coil turns in the secondary coil 118. The secondary-side current iSECONDARY is a direct function of the secondary voltage VS and the impedance of diode 120, capacitor 122, and load 104. Diode 120 allows the secondary-side current iSECONDARY to flow in one direction. The secondary-side current iSECONDARY charges capacitor 122, and capacitor 122 maintains an approximately DC voltage VLOAD across load 104. Waveforms 123 depict exemplars of control signal CS1, primary-side current iPRIMARY, and secondary-side current iSECONDARY. It is commonly assumed that the secondary-side current iSECONDARY rises virtually instantaneously after the primary-side winding 114 stops conducting the primary-side current iPRIMARY.
Since the control signal CS1 generated by the controller 106 controls the primary-side current iPRIMARY, and the primary-side current iPRIMARY controls the voltage VP across the primary coil 114, the energy transfer from the primary coil 114 to the secondary coil 118 is controlled by the controller 106. Thus, the controller 106 controls the secondary-side current iSECONDARY.
The controller 106 operates the switching power converter 110 in a certain mode, such as quasi-resonant mode. In quasi-resonant mode, the control signal CS1 turns switch 112 ON at a point in time that attempts to minimize the voltage across switch 112, and, thus, minimize current through switch 112. Controller 106 generates the control signal CS1 in accordance with a sensed primary-side current iPRIMARY—SENSE, obtained via signal iLINK—SENSE from link current sense path 126.
To attempt to deliver a known amount of power to the load 104, the controller 106 can determine the amount of power delivered to the load 104 by knowing the values of the secondary-side voltage VS and the secondary-side current iSECONDARY. The controller 106 can derive the secondary-side voltage VS from the primary-side voltage VP in accordance with VP=N·VS, as previously discussed. The controller 106 determines the value of the secondary-side current iSECONDARY by monitoring the value of iSECONDARY—SENSE, which is a scaled version of the secondary-side current iSECONDARY with a scaling factor of M. “M” is a number representing fractional ratio of the secondary-side current iSECONDARY to the secondary-side sense current iSECONDARY—SENSE. Thus, the power PLOAD delivered to the load 104 is PLOAD=VP/N·M·iSECONDARY—SENSE.
However, directly sensing the secondary-side current iSECONDARY generally requires an opto-coupler or some other relatively expensive component to provide connectivity to the secondary-side of transformer 116.